Centre of Gravity 117 



slightest disturbance, however, is enough to cause the 

 figure to revolve rapidly around the pin, and to continue 

 swinging till it comes to rest in the same position as 

 formerly, i.e. with the centre of gravity vertically 

 underneath the pinhole. Thus a body supported at 

 one point vertically below its centre of gravity is in 

 a condition of unstable equilibrium. 



72. Familiar examples of bodies in various con- 

 ditions of equilibrium may be noted, e.g. a flower-vase 

 with a long stem and a small foot will remain balanced 

 when filled with flowers. A slight jar, however, is 

 generally sufficient to upset it. The centre of gravity 

 of the vase and flowers is readily disturbed to an extent 

 which suffices to cause the foot of the perpendicular 

 from their joint centre of gravity to fall without the 

 base of the vase, with the result mentioned above. 

 The student will find it an interesting exercise to con- 

 sider some more examples of bodies known to be easily 

 knocked over, and to show that a similar explanation 

 holds good in each case. 



QUESTIONS 



1. Why does a lead-pencil roll down the sloping lid of a desk ? 



2. Why does a man balancing a walking-stick upon the end of 

 his forefinger keep the end of his finger in almost constant motion ? 



3. Which will overbalance more readily a waggon loaded with 

 hay, or a similar waggon loaded with ironmongery ? 



4. Sketch what you consider a good shape for a saucepan that 

 cannot be easily overturned. Give reasons for your opinion. 



5. A common toy consists of a small man-like figure to the 

 lower end of which is attached one-half of a spherical leaden bullet. 

 The toy always comes to rest with the figure in an upright position, 

 however it may be displaced. Why ? 



